Granite
Granite ( /ˈɡrænɪt/) is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar. The term "granitic" means granite-like and is applied to granite and a group of intrusive igneous rocks with similar textures and slight variations in composition and origin. These rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids. The extrusive igneous rock equivalent of granite is rhyolite. Granite is nearly always massive (i.e., lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history. The average density of granite is between 2.65 and 2.75 g/cm3 (165 and 172 lb/cu ft), its compressive strength usually lies above 200 MPa, and its viscosity near STP is 3–6·1019 Pa·s. The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F); it is strongly reduced in the presence of water, down to 650 °C at a few kBar pressure. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present. Origin Granite has a felsic composition and is more common in continental crust than in oceanic crust. They are crystallized from felsic melts which are less dense than mafic rocks and thus tend to ascend toward the surface. In contrast, mafic rocks, either basalts or gabbros, once metamorphosed at eclogite facies, tend to sink into the mantle beneath the Moho. Granization An old, and largely discounted process, granitization states that granite is formed in place through extreme metasomatism by fluids bringing in elements, e.g. potassium, and removing others, e.g. calcium, to transform a metamorphic rock into a granite. This was supposed to occur across a migrating front. After more than 50 years of studies, it becomes clear that granitic magmas have separated from their sources and experienced fractional crystallization during their ascent toward the surface. On the other hand, granitic melts can be produced in place through the partial melting of metamorphic rocks by extracting melt-mobile elements such as potassium and silicon into the melts but leaving others such as calcium and iron in granulite residues. Once a metamorphic rock is melted, it becomes a kind of migmatites which are composed of leucosome and melanosome. Category:Granite Category:Felsic rocks Category:National symbols of Finland Category:Plutonic rocks Category:Sculpture materials Category:Symbols of Wisconsin Category:Industrial minerals